This invention relates to aqueous two-component polyurethane systems, to a process for producing them, and to their use for the production of coatings having an enhanced impact strength and good resistance to solvents.
Environmental questions play an important part in surface technology. One particularly pressing problem is to achieve a reduction in the amounts of organic solvents which are used for lacquers and coating materials.
For chemically crosslinking polyurethane lacquers, which on account of their outstanding properties are of considerable importance in the coating sector, it has not been possible hitherto to dispense with the use of organic solvents. The use of water instead of organic solvents in two-component polyurethane lacquers based on polyisocyanates comprising free isocyanate has hitherto seemed impossible, since it is known that isocyanate groups not only react with alcoholic hydroxyl groups, but also react with water. Furthermore, the concentration of active hydrogen atoms which originate from water is of course far higher than the concentration of hydroxyl groups in the organic component which reacts with NCO, so that it must be assumed from this that the main reaction which proceeds in a polyisocyanate/organic polyhydroxy compound/water ternary system is a reaction between isocyanate and water with the formation of urea and carbon dioxide, which firstly does not result in crosslinking of the organic polyhydroxy compounds and secondly results in foaming of the lacquer batch due to the formation of carbon dioxide.
It is known from EP-A 358 979 that if selected polyhydroxyl compounds based on vinyl polymers are used as co-reactants for organic polyisocyanates comprising free isocyanate groups, aqueous two-component polyurethane systems can be produced by the emulsification, in the aqueous polymer solution or dispersion, of the polyisocyanate comprising free isocyanate groups. The polyhydroxyl compounds described in EP-A 0 358 979 are preferably polymerised by a radical mechanism in organic solution and are subsequently converted, in an aqueous solution of a neutralising agentxe2x80x94mostly of ammonia or tertiary amines, into a form in which they are dissolved in water. The organic solvent can remain in the aqueous medium if need be, or can be removed by distillation.
Polymer polyols which are produced in this manner normally exhibit single-phase morphology, and therefore exhibit either thermoplastic or elastomeric properties after crosslinking and film formation with suitable hydrophobic polyisocyanates and/or polyisocyanates which have been rendered hydrophilic. A single-phase structure of a polymer polyol such as this is generally not satisfactory for the production of polyurethane films which exhibit the properties of a thermoplastic elastomer which possesses both excellent toughness properties and a high level of strength.
It has been shown that polymer polyol systems of two-phase or multi-phase structure, in combination with suitable polyisocyanates, result in polyurethane coatings with a high level of toughness, particularly at low temperatures.
The present invention therefore relates to two-component polyurethane coating media, which contain, as a binder vehicle component, a) a polyol component consisting of at least two polymer polyols, wherein the first is present as a discrete phase and as an elastic component a1) dispersed in a second thermoplastic component a2), and b) a polyisocyanate component which has a viscosity of at the most 10,000 mPa.s and which consists of at least one organic polyisocyanate, in amounts corresponding to an NCO/OH equivalent ratio from 0.2:1 to 5:1, characterised in that component a) constitutes an aqueous solution and/or dispersion of a mixture of at least two vinyl polymer polyols of the aforementioned type in which polyisocyanate component b) is present in emulsified or solubilised form.
Component a) is a polyol component which consists of a mixture of at least two polyols a1) and a2) which are based on vinyl polymers. Polyols a1) and a2) are not chemically bonded to each other before they are crosslinked with isocyanate component b).
Polyol a1) is an elastomer component which contains hydroxyl groups, sulphonate and/or carboxylate groups, preferably carboxylate groups, and which optionally contains sulphonic acid and/or carboxyl groups, preferably carboxyl groups. Component a1) comprises polymers of olefinically unsaturated monomers, which preferably have a (number average) molecular weight Mn as determined by the gel permeation chromatography method of 500 to 500,000 g/mol, particularly 1000 to 200,000 g/mol (with respect to the uncrosslinked constituents), a hydroxyl number of 8 to 264, preferably 16 to 198 mg KOH/g solid resin, and an acid number (with respect to the sum of the un-neutralised and neutralised acid groups) of 0 to 100, preferably 3 to 50 mg KOH/g solid resin. Elastomer component a1) has a glass transition temperature (as measured by the DSC or the DMA method) of 0xc2x0 C. at most, preferably of xe2x88x9210xc2x0 C. at most.
Thermoplastic constituent a2) is likewise a polyol which contains hydroxyl groups, sulphonate and/or carboxylate groups, preferably carboxylate groups, and which optionally contains sulphonic acid and/or carboxyl groups, preferably carboxyl groups. Component a2) is likewise a polymer of olefinically unsaturated monomers, which preferably has a (number average) molecular weight Mn as determined by the gel permeation chromatography method of 500 to 500,000 g/mol, particularly 1000 to 200,000 g/mol (with respect to the uncrosslinked constituents), a hydroxyl number of 16 to 264, preferably 33 to 198 mg KOH/g solid resin, and an acid number (with respect to the sum of the un-neutralised and neutralised acid groups) of 3 to 100, preferably 5 to 50 mg KOH/g solid resin. Thermoplastic component a2) has a glass transition temperature of at least 0xc2x0 C., preferably of at least +10xc2x0 C.
Elastomer components a1) which are synthesised from the following comonomer components are quite particularly preferred:
wherein the sun of the percentages by weight of a1 a) to a1 e) is 100 and wherein the glass transition temperature is below xe2x88x9210xc2x0 C.
Thermoplastic components a2) which are synthesised from the following comonomer components are quite particularly preferred:
wherein the sum of the percentages by weight of a2 a) to a2 e) is 100 and wherein the glass transition temperature is above +10xc2x0 C.
Elastomer component a1) and thermoplastic component a2) are preferably mixed in ratios by weight from 10:90 to 60:40 (with respect to the solid resin).
The present invention also relates to a process for producing a two-component polyurethane coating medium which contains, as a binder vehicle component, a) a polyol component consisting of at least two polymer polyols, wherein the first is present as a discrete phase and as an elastic component a1) dispersed in a second thermoplastic component a2), and b) a polyisocyanate component which has a viscosity of at the most 10,000 mPa.s and which consists of at least one organic polyisocyanate, in amounts corresponding to an NCO/OH equivalent ratio from 0.2:1 to 5:1, characterised in that component a) constitutes an aqueous solution and/or dispersion of a mixture of at least two vinyl polymer polyols of the aforementioned type in which polyisocyanate component b) is present in emulsified or solubilised form.
Polymer components a1) and a2), which comprise hydroxyl groups, are produced by radical polymerisation processes known in the art, in an organic or in an aqueous phase. These polymers are preferably produced by a process comprising emulsion polymerisation by a radical mechanism in an aqueous medium.
It is possible to employ continuous or discontinuous polymerisation processes. Discontinuous processes include batch and feed processes, wherein feed processes are preferred. In a feed process, water on its own or with a portion of an anionic emulsifier, optionally admixed with a non-ionic emulsifier and with a portion of the monomer mixture, is placed in a vessel and is preheated to the polymerisation temperature, polymerisation is initiated by a radical mechanism if a monomer batch is used, and the remaining monomer mixture, together with an initiator mixture and the emulsifier, is added over the course of 1 to 10 hours. preferably 3 to 6 hours, The batch is optionally subsequently post-activated again in order to effect polymerisation to give a conversion of at least 99%.
Polymers a1) and a2), which contain hydroxyl groups, can be produced by employing techniques of metered addition such that core-shell polymers are produced (see M. J. Devon et al., J. Appl. Polym. Sci. 39 (1990) pages 2119-2128, for example). In the course of this procedure, the core can be made hard and hydrophobic and the shell can be made soft and hydrophilic, for example. The converse structure, comprising a soft core and a harder shell, is also possible.
The emulsifiers used here are of an anionic and/or non-ionic nature. Amongst emulsifiers of an anionic nature, those which contain carboxylate groups, or sulphate, sulphonate, phosphate or phosphonate groups can be used. Emulsifiers containing sulphate, phosphate or phosphonate groups are preferred. The emulsifiers may be high molecular weight or low molecular weight substances. The latter are described in DE-A 3 806 066 and DE-A 1 953 349, for example.
The preferred anionic emulsifiers are those which have been neutralised with ammonia or amines. Emulsifiers which are particularly preferred are those which are synthesised from long-chain alcohols or substituted phenols, or from ethylene oxide chains comprising degrees of polymerisation between 2 and 100 and terminal monosulphuric acid ester groups or phosphoric acid mono- and diester groups. Ammonia is generally used as the neutralising agent here, and can be added to the emulsion batch on its own or in any mixtures.
Suitable non-ionogenic emulsifiers, which are mostly used in combination with the aforementioned anionic emulsifiers, include the reaction products of aliphatic, araliphatic, cycloaliphatic or aromatic carboxylic acids, alcohols, phenol derivatives or amines with epoxides, such as ethylene oxide for example. Examples thereof include the reaction products of ethylene oxide with carboxylic acids, such as lauric acid, stearic acid, oleic acid, the carboxylic acids of castor oil, or abietic acid for example, with longer-chain alcohols such as oleyl alcohol, lauryl alcohol or stearyl alcohol, with phenol derivatives such as substituted benzyl or phenylphenols or nonylphenol, and with longer-chain amines such as dodecylamine and stearylamine for example. The reaction products with ethylene oxide are oligo- or polyethers comprising degrees of polymerisation between 2 and 100, preferably from 5 to 50. These emulsifiers are added in amounts of 0.1 to 10% by weight with respect to the mixture of the monomers.
Co-solvents which can optionally be used in conjunction include both water-soluble and water-insoluble solvents. Examples thereof include aromatic compounds such as benzene, toluene, xylene, chlorobenzene, esters such as ethyl acetate or butyl acetate, ether esters such as methyl glycol acetate, ethyl glycol acetate, methoxypropylxe2x80x94acetate or methoxybutyl acetate, ethers such as butyl glycol, tetrahydrofuran, dioxane, ethyl glycol ethers, ethers of diglycol or ethers of dipropylene glycol, ketones such as acetone, methyl ethyl ketone or methyl isobutyl ketone, trichloromonofluorethane, or cyclic amides such as N-methylpyrrolidone or N-methylcaprolactam.
Radical-initiated polymerisation can be initiated by water-soluble or water-insoluble initiators or initiator systems which have half-lives for radical decomposition between 0.01 and 400 minutes at temperatures from 10xc2x0 C. to 100xc2x0 C. In general, polymerisation is conducted in an aqueous emulsion within said temperature range, preferably between 30 and 90xc2x0 C., under a pressure of 103 to 2xc3x97104 mbar, wherein the exact polymerisation temperature depends on the type of initiator. The initiators are generally used in amounts of 0.05 to 6% by weight with respect to the total amount of monomer.
Examples of suitable initiators include water-soluble and water-insoluble azo compounds such as azoisobutyronitrile or 4,4xe2x80x2-azo-bis-(4-cyanopentanoic acid), and inorganic and organic peroxides such as dibenzoyl peroxide, t-butyl perpivalate, t-butyl-per-2-ethyl hexanoate, t-butyl perbenzoate, t-butyl hydroperoxide, di-t-butyl peroxide, cumene hydroperoxide, dicyclohexyl- and dibenzyl peroxydicarbonates, the sodium, potassium or ammonium salts of peroxydisulphuric acid, and hydrogen peroxide. Peroxydisulphates and hydrogen peroxide are often used in combination with reducing agents such as the sodium salt of formamidinesulphinic acid (Rongalit C), ascorbic acid or polyalkylene polyamines. In general, a considerable reduction in polymerisation temperature is thereby achieved.
Customary regulators can be used in order to regulate the molecular weight of the polymers, such as n-dodecylmercaptan, t-dodecylmercaptan, diisopropylxanthogen disulphide, di(methylenetrimethylolpropane)xanthogen disulphide and thioglycol. It is also possible to use allyl compounds such as the dimer of xcex1-methylstyrene. These substances are added at most in amounts of 3% by weight with respect to the monomer mixture.
After the completion of polymerisation, the polymers which are present in aqueous dispersion are optionally treated with neutralising agents to obtain degrees of neutralisation from 10 to 150% (calculated), preferably 30 to 100%. Inorganic bases, ammonia or amines are added as neutralising agents for this purpose. Sodium hydroxide or potassium hydroxide can be used as inorganic bases, for example. Apart from ammonia, trimethylamine, triethylamine, dimethylethanolamine, methyl-diethanolamine, triethanolamine, etc. can be used as amines. The neutralising agents can be used either in a stoichiometric deficit or excess.
It should be noted, however, particularly when a stoichiometric excess of neutralising agent is used, that a considerable increase in the viscosity of the polymer can occur due to the polyelectrolyte character of the system.
The co-solvents which are optionally used can remain in the aqueous dispersion in amounts up to about 20% by weight with respect to the aqueous phase. If need be, however, co-solvents can be removed by distillation after polymerisation.
Polymer dispersions a1) and a2) generally have solids contents of 20 to 60% by weight, preferably 30 to 50% by weight, viscosities of 10 to 105, preferably 10 to 104 mPa.s at 23xc2x0 C., and pH values of 5 to 10, preferably 6 to 9. They are preferably mixed in ratios by weight ranging from 10:90 to 60:40 (solid resins a1:a2).
The average particle diameter of each component which is present in the dispersion (as measured by means of laser correlation spectroscopy) is generally between 50 and 500 nm, preferably 80 to 250 nm.
Polyisocyanate component b) comprises any organic polyisocyanate which contains aliphatically, cycloaliphatically, araliphatically and/or aromatically bonded free isocyanate groups, and which is liquid at room temperature. Polyisocyanate component b) generally has a viscosity of at the most 10,000, preferably at the most 1000 mPa.s at 23xc2x0 C. Polyisocyanate component b) most preferably comprises polyisocyanates or polyisocyanate mixtures which exclusively contain aliphatically and/or cycloaliphatically bonded isocyanate groups, an (average) NCO functionality between 2.2 and 5.0 and a viscosity at 23xc2x0 C. of at the most 500 mPa.s.
The polyisocyanates may optionally be used in admixture with small amounts of inert solvents in order to reduce the viscosity to a value within said ranges. The amount of solvents of this type is calculated so that up to 20% by weight solvent, with respect to the amount of water, is present in the coating medium according to the invention which is finally obtained, wherein the solvent which may still be present in the polymer dispersions or solutions is included in this calculation. Examples of solvents which are suitable for use as additives to the polyisocyanates include aromatic hydrocarbons, such as xe2x80x9csolvent naphthaxe2x80x9d for example, and also include solvents of the type cited above by way of example.
Polyisocyanates which are particularly suitable as component b) are what are termed xe2x80x9clacquer polyisocyanatesxe2x80x9d containing aromatically or (cyclo)aliphatically bonded isocyanate groups, wherein the last-mentioned aliphatic polyisocyanates are particularly preferred, as stated above.
Polyisocyanates which have been rendered (partly) hydrophilic are quite particularly preferred.
Examples of xe2x80x9clacquer polyisocyanatesxe2x80x9d which are particularly suitable include those based on hexamethylene diisocyanate or on 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (IPDI) and/or bis-(isocyanatocyclohexyl)-methane, particularly those which are based exclusively on hexamethylene diisocyanate.
xe2x80x9cLacquer polyisocyanatesxe2x80x9d based on these diisocyanates should be understood to include the derivatives of these diisocyanates which are known in the art and which contain biuret, urethane, uretdione and/or isocyanurate groups, and which following their production have been freed in a known manner, preferably by distillation, from excess diisocyanate starting material down to a residual content of less than 0.5% by weight. The aliphatic polyisocyanates which are preferably used according to the invention include polyisocyanates based on hexamethylene diisocyanate and which contain biuret groups, and which comply with the above criteria, such as those which can be obtained, for example by the processes described in U.S. Pat. No. 3,124,605, U.S. Pat. No. 3,358,010, U.S. Pat. No. 3,903,126, U.S. Pat. No. 3,903,127 or U.S. Pat. No. 3,976,622, and which consist of mixtures of N,Nxe2x80x2,Nxe2x80x3-tris-(6-isocyanatohexyl)-biuret with subsidiary amounts of higher homologues thereof, and also include cyclic trimers of hexamethylene diisocyanate which comply with the above criteria, such as those which can be obtained according to U.S. Pat. No. 4,324,879 and which essentially consist of N,Nxe2x80x2,Nxe2x80x3-tris-(6-isocyanatohexyl) isocyanurate in admixture with subsidiary amounts of higher homologues thereof. Polyisocyanates which comply with the above criteria and which are particularly preferred are mixtures of polyisocyanates which contain uretdione and/or isocyanurate groups and which are based on hexamethylene diisocyanate, such as those which are formed by the catalytic oligomerisation of hexamethylene diisocyanate using trialkyl phosphines. The last-mentioned mixtures preferably have a viscosity at 23xc2x0 C. of at the most 500 mPa.s and an NCO functionality between 2.2 and 5.0.
The aromatic polyisocyanates which are also suitable according to the invention, but which are less preferred, comprise xe2x80x9clacquer polyisocyanatesxe2x80x9d in particular which are based on 2,4-diisocyanatotoluene or on industrial mixtures thereof with 2,6-diisocyanatotoluene or which are based on 4,4xe2x80x2-diisocyanatodiphenylmethane or mixtures thereof with isomers and/or higher homologues thereof. Examples of aromatic lacquer polyisocyanates of this type include isocyanates which contain urethane groups, such as those which can be obtained by the reaction of excess amounts of 2,4-diisocyanatotoluene with polyhydric alcohols such as trimethylolpropane and subsequent removal by distillation of the unreacted excess of diisocyanate. Examples of other aromatic lacquer polyisocyanates include the trimers of the monomeric diisocyanates cited as examples, i.e. corresponding isocyanatoisocyanurates, which have likewise been freed, preferably by distillation following their production, from excess monomeric diisocyanates.
Any polyisocyanates which can be dispersed in water and which contain aliphatically, cycloaliphatically, araliphatically and/or aromatically bonded isocyanate groups can be used as crosslinking agents for the binder vehicle dispersions according to the invention.
Example of suitable polyisocyanates include polyisocyanates which are ionically modified, e.g. those which contain carboxylate groups and optionally polyether units, of the type cited in EP-A 510 438 and EP-A 548 669, polyisocyanates which contain sulphonate groups, of the type cited in EP-A 703 255, or polyisocyanates which contain phosphate or phosphonate groups, such as those described in WO 97/31960.
However, the crosslinking agent components which are preferably used for the binder vehicle dispersions according to the invention are polyisocyanates which are purely non-ionic and which are hydrophilically modified by reaction with polyethylene oxide polyether alcohols. Polyisocyanates such as these are known from EP-A 206 059, EP-A 516 277, EP-B 540 985, EP-A 645 410, EP-A 680 983, U.S. Pat. No. 5,200,489 and from German Patent Application 19822891.0, for example. Non-ionic polyisocyanates which have been rendered hydrophilic and which are also suitable are the polyisocyanate preparations which contain special emulsifiers comprising diisocyanates and monofunctional polyether alcohols and which are cited in EP-B 486 881.
Water-dispersible polyisocyanates which are particularly preferred are the aforementioned polyisocyanates which are hydrophilically modified by means of polyethylene oxide polyether alcohols and which exclusively comprise aliphatically and/or cycloaliphatically bonded isocyanate groups. Water-dispersible polyisocyanates which are quite particularly preferred are those of the aforementioned type which have a uretdione and/or isocyanurate structure and which are based on 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclo-hexane (isophorone diisocyanate, IPDI), and/or 4,4xe2x80x2diisocyanatodicyclohexyl-methane.
In addition, polyisocyanate component b) can consist of any mixtures of the polyisocyanates cited as examples.
In order to produce the ready-to-use coating media, polyisocyanate component b) is emulsified in the aqueous dispersion of polymers a), whereupon the dissolved or dispersed polymer at the same time performs the function of an emulsifier or of a reactive component for the added polyisocyanate.
Mixing throughout can be effected simply by stirring at room temperature. The amount of polyisocyanate component is calculated so that an NCO/OH equivalent ratio of 0.2:1 to 5:1, preferably 0.5:1 to 2:1, results with respect to the isocyanate groups of component b) and the alcoholic hydroxyl groups of component a). Before adding polyisocyanate component b), customary adjuvant substances and additives can be added to polymer component a), i.e. to the dispersion or solution of the polymers. Examples of adjuvant substances and additives such as these include anti-foaming agents, flow enhancers, pigments, dispersion agents for pigment distribution and the like.
According to the invention, the polyol components crosslink with the polyisocyanates in such a way that fixation of the elastomer phase and of the thermoplastic phase (a2) is effected, whereby mixing which is too extensive is prevented. This is demonstrated in the examples with reference to the separately detectable glass transition temperatures.
The present invention further relates to the use of the coating media according to the invention. They are suitable for practically all areas of use in which solvent-containing, solvent-free or other types of aqueous paint and coating systems which possess an enhanced property profile are currently employed, e.g. the coating of practically all mineral building materials, such as lime-bonded or cement-bonded plaster, surfaces which contain gypsum, fibre-reinforced cement building materials, concrete; lacquer-coating and sealing of wood and timber materials such as chipboard and wood fibre board and also of paper; lacquer-coating and sealing of metallic surfaces; coating and lacquering of road surfaces containing asphalt and bitumen; lacquering and sealing of diverse plastics surfaces; coating of leather and textiles. They are also suitable for the all-over adhesive bonding of diverse materials, wherein different materials and materials of the same type can be bonded to each other.
The coating of plastics surfaces is particularly preferred. There is a requirement here for coating systems which exhibit an enhanced impact strength even at low temperatures (down to xe2x88x9220xc2x0 C.) whilst exhibiting a good resistance to solvents and chemicals at the same time.
After application to the substrate concerned, curing or crosslinking of the two-component system can be effected at temperatures from 5 to 300xc2x0 C., preferably between room temperature and 200xc2x0 C.